On the Plutinos and Twotinos of the Kuiper belt

We illuminate dynamical properties of Kuiper belt objects ( KBOs) in the 3 : 2 ( Plutino) and 2 : 1 ("Two-tino'') Neptunian resonances within the model of resonant capture and migration. We analyze a series of numerical integrations, each involving the four migratory giant planets and 400 test particles distributed throughout trans-Neptunian space, to measure the efficiencies of capture into the 3 : 2 and 2: 1 resonances, the efficiencies of capture into Kozai-type secular resonances, and the libration centers and amplitudes of resonant particles, all as functions of the migration speed. We synthesize instantaneous snapshots of the spatial distribution of similar to10(4) resonant KBOs, from which we derive the longitudinal variation of the sky density of each resonant family. Twotinos cluster +/-75degrees away from Neptune's longitude, while Plutinos cluster +/-90degrees away. Such longitudinal clustering persists even for surveys that are not volume limited in their ability to detect resonant KBOs. Remarkably, between -90degrees and -60degrees of Neptune's longitude we find the ratio of sky densities of Twotinos to Plutinos to be nearly unity, despite the greater average distance of Twotinos, assuming the two resonant populations are equal in number and share the same size, albedo, and inclination distributions. We couple our findings to observations to crudely estimate that the intrinsic Twotino population is within a factor of similar to3 of the Plutino population. Most strikingly, the migration model predicts a possible asymmetry in the spatial distribution of Twotinos: more Twotinos may lie at longitudes behind that of Neptune than ahead of it. The magnitude of the asymmetry amplifies dramatically with faster rates of migration and can be as large as similar to300%. A differential measurement of the sky density of 2 : 1 resonant objects behind and in front of Neptune's longitude would powerfully constrain the migration history of that planet.